High frequency circuits used for radio communication and the like include matching circuits for realizing high gain at operating frequencies. The addition of a matching circuit enables impedance matching between a transistor and a load, so power reflection does not occur between them. As a result, good characteristics are obtained (in the case of an amplifier, high gain is obtained).
In addition to capacitors and resistors, matching circuits include transmission lines which realize inductance components. However, if a transmission line is used, it is not easy to adjust an inductance value (because an adjustment of an inductance value requires, for example, the process of performing design and trial manufacture again). Accordingly, a change in operating frequency causes a deterioration in performance.
Therefore, variable inductor circuits which can electrically adjust inductance values (which may also be referred to as active inductors) are used.
Japanese Laid-open Patent Publication No. 2012-165435
Japanese Laid-open Patent Publication No. 08-181571
Y. Yu et al., “A Compact Wideband CMOS Low Noise Amplifier With Gain Flatness Enhancement”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 45, No. 3, pp. 502-509, 2010
With conventional variable inductor circuits, however, resonance occurs at high frequencies and it is difficult to adjust inductance values at high frequencies. With the conventional variable inductor circuits, for example, resonance occurs at frequencies higher than and equal to 10 GHz. It is difficult to realize a variable inductor circuit which operates even at comparatively high frequencies like those of millimeter waves higher than and equal to 30 GHz.